[RISCV] Add llvm/test/Transforms/VectorCombine/RISCV/lit.local.cfg

This directory was missing a lit.local.cfg which was causing some build
bots to fail when #65706 was comitted.

Author: michaelmaitland

llvm/test/Transforms/VectorCombine/RISCV/lit.local.cfg

@@ -0,0 +1,4 @@
+config.suffixes = [".ll"]
+
+if not "RISCV" in config.root.targets:
+    config.unsupported = True